1. Field of the Invention
The present invention relates to an image display device and an image display method. In particular, the invention relates to an image display device having an image display medium whose display density is changed by transfer of charged particle groups according to an electric field formed by a voltage applied between a pair of substrates of the image display medium, and an image display method in such an image display device.
2. Description of the Related Art
Conventionally, as an image display medium (display device) where repetitive rewriting is enabled, image display media which are configured such that colored particles such as fine particle toner are sealed between a pair of substrates have been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-347483, 2001-3383 and 2001-312225).
These media are configured such that two types of charged particle groups whose colors and charging characteristics are different are sealed between a transparent display substrate and a rear substrate which are opposed to each other with a space therebetween. The two types of charged particle groups are respectively charged with opposite polarities. By applying electric field between the display substrate and the rear substrate, the two kinds of charged particle groups transfer (move) between the substrates, and display density is thereby changed and displaying is performed. In this image display medium, by applying a voltage between the substrates according to image information, a clear image can be displayed with high contrast.
In the above conventional technique, however, when the temperature of the image display medium rises due to a rise in environmental temperature, the charge amount of the charged particles changes due to changes in characteristics of the charged particles, outgas generated from various members structuring the image display medium, changes in surface shape and surface state of the charged particles caused by bumping of the charged particles due to repetitive rewriting, and the like. Particularly, when the temperature of the charged particles rises to a predetermined temperature or more, it is known that the charge amount is extremely reduced.
When the charge amount of the charged particles is reduced, it becomes difficult for the charged particle groups to transfer between the substrates when a display driving voltage for displaying an image on the image display medium is applied between the substrates. Such reduction in response to the voltage applied to the charged particle groups gives rise to the problem of reduction in display density.